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Fair value measurements (Details 2) (USD $) | Dec. 31, 2013 | Jun. 30, 2013 | Dec. 31, 2012 | Jun. 30, 2012
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Concentration risks, percentage [Abstract] |  |  |  |  |  |
Percentage investment in common stock of mid-cap companies | 29.00% |  |  | 28.00% |  |
Percentage investment in common stock of small-cap companies | 28.00% |  |  | 28.00% |  |
Percentage investment in common stock of large-cap companies | 28.00% |  |  | 29.00% |  |
Percentage in fixed-income investments | 15.00% |  |  | 15.00% |  |
Assets, fair value disclosure, nonrecurring |  |  | $ 9,700,000 |  |  | $ 2,500,000
Fair value, measurements, recurring [Member] |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets: | 93,657,000 |  |  | 101,443,000 |  |
Liabilities: | 7,483,000 |  |  | 6,255,000 |  |
Fair value, measurements, recurring [Member] | Commodity derivative instruments liabilities [Member] |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Liabilities: | 7,483,000 |  |  |  |  |
Fair value, measurements, recurring [Member] | Interest rate derivative instruments |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Liabilities: |  |  |  | 6,255,000 |  |
Fair value, measurements, recurring [Member] | Money market funds |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets: | 19,227,000 |  |  | 24,240,000 |  |
Fair value, measurements, recurring [Member] | Insurance contract |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets: | 62,370,000 | [1] |  | 48,898,000 | [2] |
Fair value, measurements, recurring [Member] | Mortgage-backed securities |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets: | 8,193,000 |  |  | 8,195,000 |  |
Fair value, measurements, recurring [Member] | U.S. Treasury securities |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets: | 1,917,000 |  |  | 1,806,000 |  |
Fair value, measurements, recurring [Member] | Commodity derivative instruments |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets: | 1,950,000 |  |  | 18,304,000 |  |
Fair value, measurements, recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets: | 0 |  |  | 0 |  |
Liabilities: | 0 |  |  | 0 |  |
Fair value, measurements, recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Commodity derivative instruments liabilities [Member] |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Liabilities: | 0 |  |  |  |  |
Fair value, measurements, recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Interest rate derivative instruments |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Liabilities: |  |  |  | 0 |  |
Fair value, measurements, recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Money market funds |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets: | 0 |  |  | 0 |  |
Fair value, measurements, recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Insurance contract |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets: | 0 | [1] |  | 0 | [2] |
Fair value, measurements, recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Mortgage-backed securities |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets: | 0 |  |  | 0 |  |
Fair value, measurements, recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Treasury securities |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets: | 0 |  |  | 0 |  |
Fair value, measurements, recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Commodity derivative instruments |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets: | 0 |  |  | 0 |  |
Fair value, measurements, recurring [Member] | Significant Other Observable Inputs (Level 2) |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets: | 93,657,000 |  |  | 101,443,000 |  |
Liabilities: | 7,483,000 |  |  | 6,255,000 |  |
Fair value, measurements, recurring [Member] | Significant Other Observable Inputs (Level 2) | Commodity derivative instruments liabilities [Member] |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Liabilities: | 7,483,000 |  |  |  |  |
Fair value, measurements, recurring [Member] | Significant Other Observable Inputs (Level 2) | Interest rate derivative instruments |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Liabilities: |  |  |  | 6,255,000 |  |
Fair value, measurements, recurring [Member] | Significant Other Observable Inputs (Level 2) | Money market funds |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets: | 19,227,000 |  |  | 24,240,000 |  |
Fair value, measurements, recurring [Member] | Significant Other Observable Inputs (Level 2) | Insurance contract |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets: | 62,370,000 | [1] |  | 48,898,000 | [2] |
Fair value, measurements, recurring [Member] | Significant Other Observable Inputs (Level 2) | Mortgage-backed securities |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets: | 8,193,000 |  |  | 8,195,000 |  |
Fair value, measurements, recurring [Member] | Significant Other Observable Inputs (Level 2) | U.S. Treasury securities |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets: | 1,917,000 |  |  | 1,806,000 |  |
Fair value, measurements, recurring [Member] | Significant Other Observable Inputs (Level 2) | Commodity derivative instruments |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets: | 1,950,000 |  |  | 18,304,000 |  |
Fair value, measurements, recurring [Member] | Significant Unobservable Inputs (Level 3) |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets: | 0 |  |  | 0 |  |
Liabilities: | 0 |  |  | 0 |  |
Fair value, measurements, recurring [Member] | Significant Unobservable Inputs (Level 3) | Commodity derivative instruments liabilities [Member] |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Liabilities: | 0 |  |  |  |  |
Fair value, measurements, recurring [Member] | Significant Unobservable Inputs (Level 3) | Interest rate derivative instruments |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Liabilities: |  |  |  | 0 |  |
Fair value, measurements, recurring [Member] | Significant Unobservable Inputs (Level 3) | Money market funds |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets: | 0 |  |  | 0 |  |
Fair value, measurements, recurring [Member] | Significant Unobservable Inputs (Level 3) | Insurance contract |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets: | 0 | [1] |  | 0 | [2] |
Fair value, measurements, recurring [Member] | Significant Unobservable Inputs (Level 3) | Mortgage-backed securities |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets: | 0 |  |  | 0 |  |
Fair value, measurements, recurring [Member] | Significant Unobservable Inputs (Level 3) | U.S. Treasury securities |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets: | 0 |  |  | 0 |  |
Fair value, measurements, recurring [Member] | Significant Unobservable Inputs (Level 3) | Commodity derivative instruments |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets: | $ 0 |  |  | $ 0 |  |
[1] | The insurance contract invests approximately 29 percent in common stock of mid-cap companies, 28 percent in common stock of small-cap companies, 28 percent in common stock of large-cap companies and 15 percent in fixed-income investments.
[2] | The insurance contract invests approximately 28 percent in common stock of mid-cap companies, 28 percent in common stock of small-cap companies, 29 percent in common stock of large-cap companies and 15 percent in fixed-income investments.

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